Genetic engineers experiment with altering, splicing, eliminating, and rearranging genes to obtain specific results. They use a variety of techniques such as mutagenesis, transposon insertions, selective breeding, and recombinant DNA technology to alter the genetic material of cells. The goal of such genetic modification is to make cells perform new functions, produce new substances, or block the production of a particular substance. The work of genetic engineers and scientists has resulted in many breakthroughs, including the discovery and production of insulin and interferon, which are used to treat diseases like diabetes and leukemia. Genetic engineering successes have also been seen in agricultural science. Agricultural triumphs like hybrid corn, disease-resistant grains, and higher quality livestock are all products of the principles of recombinant DNA and cloning. The following paragraphs detail some of the areas in which genetic engineers specialize. Some genetic engineers may work in more than one area.

Genetic engineers who specialize in gene therapy try to cure diseases and medical conditions by using the principles of genetic engineering. Individuals with certain diseases may have a gene that caused the disease. The goal of gene therapy is to replace disease-causing genetic material (DNA or RNA) with a healthy gene (or genes), or to make a beneficial protein. Genetic engineers undertake this process in two ways: through ex vivo (external) gene therapy or by in vivo (internal) gene therapy. In external gene therapy, genetic engineers remove some of the patient’s blood cells and place a healthy copy of the gene into these cells. They will culture, or grow, millions of these modified cells in a laboratory, and then inject them into the patient’s blood in the hope that the altered cells will build a healthy immune system with this new genetic information. Internal gene therapy is used when cells cannot be genetically engineered outside of the body. In this process, genetic engineers insert genetically modified cells into the area of the body that needs treatment with the same goal of encouraging an improvement in health.

Genetic engineers use a carrier called a vector to ensure that the genetically modified cells reach the intended area of the body without harming other cells in the process. Vectors are used in both ex vivo and ex vivo gene therapy. Viruses are often used as vectors in gene therapy. The viruses are modified so that they do not harm the individual. Genetic engineers also conduct research to find types of vectors that are more effective in transmitting genetically engineered cells during gene therapy.

Genetic engineering is used in agriculture to develop new methods of growing crops to ensure more efficient production, higher yields, and improved quality or nutritional benefits. Genetic engineers work with agricultural scientists to develop crops that are resistant to pests, drought, and plant diseases. They use biotechnology to increase the nutritional value of crops and the quality of seed stock, reduce the amount of harmful pesticides needed to grow crops, create weed-resistant plants, and produce foods that are free of allergens or toxins (such as mycotoxin, which can cause people who consume mycotoxin-contaminated crops to become very ill or even die).

Genetic engineers work in animal science to improve the health of animals and humans, increase livestock productivity, and reduce the environmental impact of agricultural production. The Biotechnology Innovation Organization (BIO) reports that animals that have been safely genetically engineered in research studies include pigs, cattle, goats, chickens, dogs, cats, fish, rats, mice, and sheep. One example of a genetically engineered animal is the Enviro-Pig. The BIO reports that "through genetic engineering, this animal emits 30 to 60 percent less phosphorous than traditional pigs fed the same conventional diet...which lessens livestock’s impact on the environment." Pigs can also be genetically engineered to produce donor tissues that are compatible to humans. In late 2015, the FDA approved the first genetically engineered animal—the AquaAdvantage salmon—for introduction into the human food supply.

Genetic engineers also help scientists develop ways to clean up pollution. A process called bioremediation is used to clean up chemicals such as paint in landfills or oil that has been accidentally spilled into soil or water. In bioremediation, bacteria or other microbes are used to break down the toxic substances into simpler compounds that are less toxic to the environment. Genetic engineers may introduce pollution-fighting genes from another species into a microbe to enhance its pollution-fighting skills or engineer the organism’s existing genes to make it more effective.